The HD 650 are a fun headphone but so much that they lose any of their audiophile credentials. They aren't as analytical as the DT 880. They aren't as spread out as the Q701. They deliver impact without whacking sound stage and detail. i find them to be a great rock can--usually better than their more neutral sibling the HD 600.
I agree with what some reviewers like Chris Martens have said--the sound of the HD 650 can be highly amp dependent. I have also found that, like Chris, the HiFiMan EF 5 hybrid amp is a great combo with them. Before Senn MAPed them, they were an excellent buy.